The present invention relates to a spherical rubber molding suitable for use as a one-piece golf ball material or as a core material for solid golf balls such as two-piece golf balls and three-piece golf balls. More specifically, the invention relates to a molded rubber material for golf balls which has a high hardness, a high resilience and a high durability. The invention additionally relates to a method of producing such a molded rubber material, and to a golf ball in which such a molded rubber material is used.
Rubber compositions for producing molded and crosslinked rubber materials are generally made of polybutadiene, zinc acrylate and an organic peroxide, and include also zinc oxide as a filler or as a vulcanizing agent or vulcanization co-accelerator. If zinc oxide is not included in the rubber composition, the molded material obtained from the composition will soften, resulting in a lower durability. Therefore, zinc oxide is often included in molded and crosslinked rubber materials for use as solid cores and the like in the production of golf balls. In particular, zinc oxide is known to function as an activator which exerts a large influence on crosslinking reactions between rubber molecules. Hence, it is likely that the quality and compounding ratio of this zinc oxide strongly affect the physical qualities of the molded and crosslinked rubber material. The degree of activity by zinc oxide varies with its particle size. For example, when fine particles of zinc oxide are used, the zinc oxide has a higher degree of activity, making it possible to readily achieve a high hardness and also improving the durability.
This is illustrated by JP 2003-126300, which discloses improved resilience and durability owing to the use of zinc oxide having an average particle size of 200 nm or less in a rubber composition for golf balls.
However, the zinc oxide is difficult to uniformly disperse in such a composition. By enhancing uniform dispersion, there is room for further improvement in reactivity and durability.
A variety of methods are known for producing fine zinc oxide particles, including dry methods, wet methods and grinding methods. For example, along dry methods based on the French process, one chemical vapor phase deposition process categorized as the electric furnace process involves melting and vaporizing metallic zinc at a low temperature of 1500° C. or below so as to oxidize it with air, then cooling to obtain zinc oxide. Among wet methods, zinc oxide can be obtained by reacting a solution of a soluble zinc salt with an alkali solution. Grinding methods include the reduction of zinc oxide having a large particle size to fine particles by physical grinding such as in a jet mill.
Moreover, JP-A 2000-191489 teaches a process for enhancing dispersibility by applying a surface treatment agent to zinc oxide which is in the form of fine particles.
However, the surface treatment in this process has the undesirable effect of lowering the degree of activity by the zinc oxide, preventing the physical improvements arising from the high activity of the fine particles from being achieved.